A diode has been used as a rectifying element in an alternator that generates power in a car. A diode is inexpensive, but has a large loss due to a large forward voltage drop. On the other hand, a MOSFET has recently come into use as a rectifying element for an alternator in place of a diode. A MOSFET can be used for synchronous rectification, in which a forward current starts to flow just above 0 V to have a low loss.
An AC of a commercial power source has a constant frequency. Then, if a MOSFET is used as a rectifying element in a power supply device using the commercial power source, ON-OFF control of the MOSFET can be performed in synchronization with the clock. However, the frequency of AC electric power generated by a coil is not constant in an alternator. This requires, if a MOSFET is used as a rectifying element in the alternator, performing ON-OFF control of the MOSFET in synchronization with the various frequencies at any given time, rather than merely in synchronization with the clock frequency as is the case where the MOSFET is used in a power supply device.
A method of using a Hall element to detect the position of the rotor in a motor for controlling a MOSFET is known as a method for performing ON-OFF control of the synchronously rectifying MOSFET in an alternator. Such a method of performing control by inputting signals from external sources such as a Hall element will hereinafter be referred to as an externally controlled type. An externally-controlled synchronously rectifying MOSFET needs to have a sensor such as a Hall element to perform complicated control by a control circuit, thereby making a rectifier of the alternator expensive.
Japanese Translation of PCT International Application Publication No. JP2011-507468 discloses a method of determining a voltage across the source and the drain of a synchronously rectifying MOSFET to control a MOSFET, as another method for performing ON-OFF control of the synchronously rectifying MOSFET in an alternator. Such a method of performing control based on an internal voltage without any signal from outside will hereinafter be referred to as an autonomous type. An autonomous synchronously rectifying MOSFET requires no sensor such as a Hall element to use a simple control circuit in general, and this allows a rectifier in an alternator to be inexpensive. In Japanese Translation of PCT International Application Publication No. JP2011-507468, a differential amplifier is used as a circuit for determining an ON or OFF state of a MOSFET using a voltage across the source and the drain of a synchronously rectifying MOSFET, and others such as a comparator can also be used.
Furthermore, the rectifier of the synchronously rectifying MOSFET in the alternator, which is disclosed in Japanese Translation of PCT International Application Publication No. JP2011-507468, is provided with a capacitor for supplying power to the control circuit used in the rectifier. Incorporating a capacitor as a power source allows to have two external terminals. This allows for having the same terminal arrangement as a diode to be used in place of a diode in the alternator.